The Mass Effect trilogy is probably among my top favorite single player games out there (I'm almost always playing Multiplayer games instead xP) Bioware really knows how to deliver an RPG experience, there's just so much delicious lore/culture in their titles that you can't help but get sucked in. I'm sure there are people out there who invest more time in relationships with a fictional crew member than they do with someone irl, hahaha!
Not only that though, I love how the mechanics promote well-played tactics/combos. I remember spending hours on the final mission of ME3 trying to beat it on Insanity with a Vanguard, it was stupidly rewarding when I finally completed it.
I kinda wish I spent more time with ME3 after beating the campaign though, I've heard a lot of great things out of the co-op mode from a friend recently.
PS. This guy has a lot of helpful ME reference stuff: [link]